1. Field of the Invention
The present invention relates to a thermal fuse employing a thermal pellet, particularly a thermal pellet type thermal fuse with means for suppressing sublimation of the thermal pellet.
2. Description of the Background Art
Thermal fuses are used to protect household and industrial electronic/electrical apparatuses from damage caused by excessive heat. The thermal fuse has a compact and sturdy structure with the capability of cutting off circuitry promptly in the event of detecting an abnormal increase in temperature in order to avoid damage of the apparatus and fire disaster. With respect to the wide range of the nominal current ratings of 0.5 A to 15 A, usage of a thermal pellet type thermal fuse employing a thermal pellet for the thermal element is recommended for applications involving a high current of 6 A and above. A typical structure of a thermal pellet type thermal fuse includes a thermal pellet that is formed by molding insulative chemical material, which is hermetically sealed in a metal casing together with a movable electrode, weak and strong springs, and the like, and has a lead output from the casing. For example, Japanese Utility Model Laying-Open No. S57-94142 discloses a thermal pellet type thermal fuse that has a compression spring arranged at the thermal pellet side with a disk therebetween in a metal casing, and a barrel inserted to form close contact at the front peripheral portion in view of the problem that a thermal pellet type thermal fuse operates erroneously at a temperature lower than a predetermined operating temperature. Japanese Utility Model Laying-Open No. S57-103647 discloses a structure in which two pressure plates sandwiching a resilient ring therebetween are disposed between a thermal pellet and a strong compression spring in a metal casing. Japanese Patent Laying-Open No. 2004-119255 discloses a structure in combination with a thermal pellet of an insulating material that does not sublime in order to suppress sublimation of the thermal pellet.
Thermal-sensing materials include materials that easily sublime such as pure chemical agents and materials that do not easily sublime such as thermoplastic resin. A thermal pellet is fabricated by granulating powder of a thermal-sensing substance, and molding the granules into a predetermined shape. In the case where the sublimation action is relatively great or where deformation by shrinking or softening readily occurs, the pellet may be deformed prior to arriving at a predetermined operating temperature, leading to erroneous operation. Particularly in a current flowing state of usage, facilitation of sublimation of the thermal pellet may be expected at an ambient temperature lower than the predetermined operating temperature. It has been desired to alleviate such detrimental factors.